1. Field of the Invention
The present invention relates to ink-jet printing control for printing by ejecting ink droplets onto print media, and more particularly to an ink-jet printing control which is capable of providing stable performance of ink droplet ejection.
2. Description of Related Art
Among conventional ink-jet printing apparatuses, there is a drop-on-demand arrangement of a shear mode type using piezoelectric ceramic material as disclosed in Japanese Unexamined Patent Publication No. 63-247051. A printing head used in this kind of ink-jet printing apparatus is shown in FIGS. 8A and 8B.
Referring to FIGS. 8A and 8B, a printing head 21 is provided with a cover plate 201 and a base plate 202 which is provided facing the cover plate 201. A part between the cover plate 201 and the base plate 202 is formed with piezoelectric material so that partitions are provided by a plurality of shear mode actuator walls 203 polarized in arrow directions F30 and F40 indicated in FIG. 8A, and an ink channel 205 and an air channel 212 are arranged alternately between each pair of shear mode actuator walls 203. One side of each shear mode actuator wall 203 has a film electrode 204, and the other side thereof has a film electrode 214.
As shown in FIG. 8B, the front end of the shear mode actuator wall 203 is provided with a nozzle plate 207 which has nozzles 206 each of which is connected with the ink channel 205, and the rear end of the shear mode actuator wall 203 is provided with a manifold part 209 which has a filler part 208 for preventing intrusion of ink from a common ink passage 213 into the air channel 212. The manifold part 209 is used for distributing ink from an ink reservoir (not shown) to each ink channel 205. Each of the electrodes 204 and 214 is covered with an insulating layer (not shown), and the electrode 214 facing the air channel 212 is connected with the ground 211. The electrode 204 forming the ink channel 205 is connected with a head driver IC (integrate circuit) 83 which applies an actuator drive signal to the electrodes 204 and 214.
In the printing head 21 structured as mentioned above, when the head driver IC 83 applies the actuator drive signal 110 having a pulse width T shown in FIG. 10 to the electrode 204, piezoelectric thickness slip deformation occurs on each shear mode actuator wall 203 to increase a volume of the ink channel 205. For instance, as shown in FIG. 9, when a positive drive voltage is applied to the electrode 204 of the ink channel 205, electric fields are produced on the shear mode actuator wall 203 in arrow directions F10 and F20, causing piezoelectric thickness slip deformation to occur on upper walls 203a and lower walls 203b of the shear mode actuator wall 203 so that the volume of the ink channel 205 is increased. At this step of operation, pressure in the ink channel 205 including a vicinal part of the nozzle 206 is decreased. This state is maintained during the pulse width T which corresponds a period of one-way propagation time T of a pressure wave in the ink channel 205. Thus, ink is supplied from the common ink passage 213 thereinto.
The one-way propagation time T of the actuator drive signal 110 indicates a period of time required for the pressure wave in the ink channel 205 to complete propagation in the longitudinal direction of the ink channel 205. Using length `L` (FIG. 8B) of the ink channel 205 in the longitudinal direction thereof and acoustic velocity `a` in ink in the ink channel 205, `T` is expressed as follows; T=L/a.
Based on the principle of pressure wave propagation, pressure in the ink channel 205 is reversed to become positive after a lapse of time T following application of the drive voltage. At the timing of pressure reversal, the drive voltage being applied to the electrode 204 of the ink channel 205 is reset to zero (0) V. Thus, the shear mode actuator wall 203 is restored to normal (FIGS. 8A and 8B), applying pressure to ink. At this step of operation, the positive pressure is added to pressure which has been produced by restoration of the shear mode actuator wall 203 to normal, so that relatively high pressure is generated in the vicinity of the nozzle 206 in the ink channel 205, thereby ejecting ink from the ink channel 205 to the outside through the nozzle 206.
In actual use, the printing head 21 is mounted on a carriage which is electrically driven by a motor to move transversely across the printable medium during the printing operation. As the motor speed changes greatly at the time of starting and ending an energization of the motor, the moving speed of the printing head 21 relative to the printable medium changes correspondingly. Further, the moving speed of the printing head 21 changes, even during the constant speed rotation of the motor, due to other causes such as friction between the carriage and a carriage shaft supporting the carriage movably thereon. As a result, even if the drive pulse signal 110 is applied at a fixed driving interval (fixed frequency), the actual driving interval varies when the transverse moving speed of the printing head relative to the printable medium changes.
According to measurement of the ink ejection speed against the driving interval of the drive pulse signal 110, as shown in FIG. 7, the ink ejection speed changes between the highest speed (9.00 m/s) at a driving interval of 110 .mu.s and the lowest speed (6.25 m/s) at another driving interval of 125 .mu.s. Thus, the maximum change in the ink ejection speed is 2.75 m/s.
The above variation in the driving interval of the actuator drive pulse signal 110 thus causes variations in the ink ejection speed of the printing head 21 due to a residual pressure wave vibration in the ink chamber 205. This results in degradation in the printing quality.